A radar system may measure the distance to a target through measurement of the round-trip time-of-flight of the radar signal to a target and return. The one-way distance, d, to a target is computed from the equation 2d=ct where c is the velocity of light and where t is the time between transmitted signals and received signals reflected from a target. Radar technology is well developed. However, an individual target may be difficult to isolate if there are many reflectors in the field of the radar system. Also, the distance calculation may be corrupted by multipath effects and limited bandwidth of the transmitted pulse. Multipath effects may be mitigated by using the time of the first return signal from a target. However, this technique is fraught with problems if the reflected signal from the desired target is hidden by signals reflected from other objects in the field of the radar system.
RFID systems are well known in the art and are used to monitor objects and places by attaching tags to objects and places to be monitored. These objects may be large and in the presence of many other reflecting objects. RF signals from a tag may be hidden by noise and larger signals reflected from other reflecting objects. Backscatter RFID readers transmit CW signals while acquiring data from tags, and thus lack the capability of ranging using measurement of time-of-flight methods. RFID tags used in modulated backscatter RFID systems are often referred to as ‘passive’ (without an internal source of power) or ‘semi-passive’ (with an internal source of power) since modulated backscatter tags do not generate radio signals and only reflect radio signals. RFID tags may also send data to a RFID reader by generating and sending radio signals. These types of RFID tags are often referred to as ‘active’ tags since they generate radio signals and contain an internal source of power. The phase of the backscattered signals from a modulated backscatter tag can be used to calculate the distance to a tag in the presence of other reflecting objects, as disclosed in U.S. patent application Ser. No. 12/840,587, titled SYSTEM AND METHOD FOR MEASUREMENT OF DISTANCE TO A TAG BY A MODULATED BACKSCATTER RFID READER, but accuracy may be degraded in a highly reflecting environment due to multipath effects. Many types of RFID systems use modulation signals with frequencies on the order of a megahertz or less and often shape waveforms to comply with radio regulations. These modulation waveforms lack nanosecond precision needed to use time-of-flight methods to measure distance to a resolution on the order of a meter or less between tags and readers.
RFID systems using time-of-flight methods to determine object location may be found in the art but these types of systems are expensive, require careful installation, use expensive tags and require precise positioning of the system components. Signal strength methods to determine tag location may also be found in the art, but these types of systems lack accuracy and precision.
Many tens of millions of RFID tags are presently in use and installations would benefit if the distance to these tags could be measured accurately in a complex radio environment.
Modulated Backscatter RFID System of the Prior Art
A modulated backscatter RFID tag transfers data from its memory to a remote reader by modulating the backscatter cross section of the tag antenna in a coded fashion, changing at a minimum from one reflecting state to another reflecting state (or between several reflecting states) in a time-wise fashion, thus coding the tag data on the time-varying backscatter cross section of the tag. A continuous wave (CW) radio signal is transmitted toward a tag by a reader. The tag modulates the reflected signal sent back to the reader thus producing a time-varying signal encoded with data from the tag. The reader then receives and decodes the modulated signal from the tag to extract the information sent by the tag. The decoding process recovers the timing of the changes in modulation states of the tag. These timings cannot be used for time-of-flight calculations since there is no absolute time reference to establish a time base for calculation. Another practical problem is that the transitions from one modulation state to the other lack the bandwidth, precision and definition in timing required for nanosecond resolution required for ranging. For example, a resolution of 1 meter in tag location requires a timing resolution of 7 nanoseconds or better. Typical RFID systems such as specified by ISO/IEC 18000-6: 2004(E) and ISO/IEC 18000-6:2004/FDAM 1:2006(E) require timings, such as rise and fall times, on the order of microseconds and are thus over 1000 times to slow. The reader also decodes the states of modulation as a function of time. The reader uses these states to recover the bit pattern, and thus data, sent by the tag.
Pulse Radar System of the Prior Art
An example of the geometry of a conventional radar system is shown in FIG. 1. The radar system transmits a RF signal which is reflected from the objects in the field of the radar and are received by the radar system. Strong multipath signals may occur from a radio path bounced from the radar system to a flat surface (ground for example), to targets, and return. A sample plot of signals is shown in FIG. 2. To measure the distance to a single desired target, the correct return signal of the many in FIG. 2 is required.
All references cited herein are incorporated herein by reference in their entireties.